it means when you are making a deal you might say that at the end
so if you like her then i can make a deal so that she will fall for you and i get 100 dollars in return other person YOU GOTTA DEAL BABY SEAL
To horrifically ruin a romantic excursion; "sealing the deal" gone sour.
"Greg sneezed over his date's boobs last night; that really dealed the seal."